Your attempted rate will depend on what type of captchas you run accross. If you are targetting some well supported types, then you'll of course get a high rate. If you are hitting targets that heavily use recaptcha, then its going to be lower.
I see you have solved roughly 500k out of 1 million. Since recpatcha accounts for about 50% of all captchas served on the web, this stat is very much inline with expectations.
Regarding your screenshot, Id like you to take a look at the info and data in the left hand column regarding the success rate for the captcha which was detected:
Here we have Drupal at 21%, Burning Board at 13% and Plogger at 16%.
So none of the ones in question have a very good solve rate and could easily come up with a no answer result.
If you really think there is a problem, just save your captchas and then do a quick visual audit by looking at the captchas in the saved captchas folder which will have the answer given saved as the filename